It is strange how Shelley's rebel spirit moves with the times. He is continually reinvented as an icon of liberation. Once declaimed by Mick Jagger at a Hyde Park pop concert, he has since been recited by Chinese students at Tiananmen Square and quoted by President Thabo Mbeki in an inauguration address in South Africa. Damned by Matthew Arnold as a Miltonic rebel angel, "beating in the void his luminous wings in vain", he was subsequently grounded by the radical journalist Paul Foot as "Red Shelley", the demonic street-fighter. James Bieri's recent two-volume biography soberly commends him as "the poet of political, religious and social dissent".

Red Shelley has also been recast as green Shelley, an early champion of our current concerns: vegetarianism, atheism, climate change and popular science. His crisp, acerbic observations on God (attached to his poem "Queen Mab") would delight Richard Dawkins and Christopher Hitchens. While his use of Herschel's astronomy, Buffon's geology and Humphry Davy's chemistry in his visionary poem "Prometheus Unbound", would probably qualify him for a Templeton prize ("expanding spiritual knowledge through science").

Almost everything about Ann Wroe's daring biography announces yet another transformation. From its new age subtitle and its startling cover image (a naked male dancer in a Blakean burst of apocalyptic sunlight) it is clear that this is a radiant, me-generation Shelley, breaking free into the contemporary zeitgeist.

In her impassioned introduction, which sets the tone for the book, Wroe argues that Shelley has been brought "too severely to earth" by critics and biographers concentrating on his political radicalism. He is a poet whose "spiritual force seems to have been largely forgotten". She has set out to recover a "metaphysical" visionary, attempting to write his life experimentally, "from the inside out ... from the perspective of the creative spirit struggling to discover its true nature". Biographical priorities are "reversed". In an ingenious dream sequence, Shelley sets sail on the opening page (but perhaps in a paper boat).

This means that Wroe abandons conventional chronology. Instead, she organises her book around a loose, magical structure of the four elements of ancient cosmology. "The narrative track is the poet's quest for truth through the steadily rarefying elements of earth, water, air and fire."

So rather than solid life story, we are given a kind of shimmering life-collage. This is a form Wroe used slightly differently in her previous book, Pilate: The Biography of an Invented Man. She takes particular themes, images or ideas and brings them together simultaneously from every stage in Shelley's career. It is an extraordinary (if not always reliable) feat of scholarship, and it produces a sense of startling immediacy that sets it apart from any previous Shelley study.

Very early on (in the section "Earth") we get this wholly unexpected snapshot of the poet. "As closely as was feasible, he stared at things. He got right down beside the plate to study pink fatty slabs of bacon or the jutting crag of a teacake. Pressed against fir trees, he inspected and licked the oozy runnels of resin. He read with his face only inches from the page, and watched tiny insects in the palm of his hand with fervent dedication."

Each of these details is lovingly assembled from biographical minutiae: a visit to Southey, a walk in a Tuscan wood, a boating expedition on the Thames, a late love-lyric. The texture of the entire book is built up from this sort of rippling, impressionist mosaic of poetry, letters, notebook drafts, fragments of essays, anecdotes, drawings and memoirs.

The effect is continually surprising. Here is the angelic poet as a monster: "Shelley could crook his fingers into talons, scratching the air, or screw his long curls grimly into a devilish horn. He could caper and scream at night on Hampstead Heath ..." Fiction? Not quite, because everything is based on the genuine reminiscences of Leigh Hunt's son, Thornton Hunt (though admittedly he was only 11 at the time).

The fascination of these composite recreations is frequently sustained for long sequences. In the second section, "Water", there is a brilliant 10-page evocation of Shelley and boats, ranging from precise details of his boyhood skiffs at Eton and his perilous yacht at Lerici, to his most visionary poetic voyages in "Alastor" and "Adonais". "Water," she writes, "marked his first escape from the densest matter, slippery and quick and translucent."

With the section "Air", there is a wonderful account of "Ode to the West Wind" and of Shelley lifted on his "soul-wings, unfolded". "Poem after poem suggested that Shelley viewed the Earth from the air ... there, directly over the mast, he glimpsed his boat, a tiny open shell with himself a shadow in the prow." Wroe remarks that his friends were "only half joking" when they presumed Shelley could actually fly, and all were puzzled that he could not swim. The blazing upward trajectory climaxes in the final section, "Fire". We vaporise through revolutionary volcanoes, hot-air balloons, Humphry Davy's "green-glowing Leyden jars", and the sexual combustion of Shelley's ecstatic autobiography, Epipsychidion. This contains "his most explicit descriptions of the soul", though in a sense these go up in self-consuming smoke.

Throughout, Wroe adopts a form of ventriloquism, taking over Shelley's voice and extending his imagery for herself. Sometimes this works superbly, but frequently it teeters on the edge of gaudy, angelic parody. "He glided like a young eagle, free, clear-sighted, while fire-thoughts streamed from his wings and his soul-shadow, like a storm passing, flashed over the Earth." Here we seem to be back with Arnold's aerodynamics, only with added jet propulsion. But Wroe also gets "inside" her Shelley by going back to his original manuscripts and notebooks, now housed in Oxford's Bodleian Library and the Huntingdon Library in California. She uses these more imaginatively than any previous scholar.

She captures the physical impact of Shelley's pen upon paper, the hundreds of drawings and doodles he made, the effect of sun, seawater or tears. Some words are scratched with a knife, others break the nib or pierce the paper surface, "stabbing and ripping", others are smudged or scrawled or repeated mantra-like. In one notebook a meteor slashes across an entire page; in another "little blank-faced devils" appear at the margins; in a third sailing ships drift like swans to paradise islands. There is even a careful drawing of what Wroe assumes (a little impetuously perhaps) to be Shelley's own penis.

But above all she uses the deletions Shelley made, actually printing the lines with their crossings-out and corrections. This gives a remarkable effect of intimacy, of being at the poet's elbow (or wing-feather) as he wrote. The poem "To Constantia", addressed to Shelley's half-sister-in-law, Claire Clairmont, with all its deletions restored, is changed from a salon lyric to a hymn to sexual arousal. The varied prefaces to Epipsychidion produce a kind of alternative dream biography. We also learn that Shelley originally wrote, "Hail to thee blithe spirit / Bird thou hardly art" - thus avoiding the wincing "never wert" that has convulsed generations of schoolchildren.

Wroe's glowing narrative immediacy is achieved at a certain cost. Some of the darker poems, such as "The Triumph of Life", seem badly under-exposed. There is too much reliance on the veracity of Victorian memoir-writers, such as Trelawny and his tall tales of naked supper parties. Often there is a novelettish aura, as descriptions take on an unnatural brightness, and facts seem to quiver and inflate towards fantasy.

Moreover, without chronology, many readers may struggle to follow the chaotic adventure story of Shelley's life - his restless travels, his serial love-affairs, his creative leaps and bounds. The poet exists only in a kind of dazzling, hypnotic present tense. In "searching for himself", Shelley's companions (notably Mary and Byron) may also remain remote and shadowy, perhaps another authentic feature of the me-generation.

Yet Shelley's "spiritual force" - that extraordinary combination of pantheism, Platonism, philandering and post-Newtonian science - is frequently reignited in Wroe's fiery upward spiral of visions and inventions. This transcendence of conventional biography is the Shelleyan gamble of Being Shelley. "If he were to settle anywhere he would sink into time, and the hours would close around him."

The result is a risky but singularly exhilarating book, which will probably outrage Shelley scholars, but also liberate a new generation of Shelley readers. If it is more of a glittering mythography than a biography, it can be relished as such. If not Shelley plain, it is certainly Shelley purl.

· Richard Holmes's Shelley: The Pursuit is published by Harper Perennial